Well, I'm thinking along the same lines as kb2yjx... Weightloss and excessive begging in an elderly cat could be due to one of several more serious underlying health problems common to cats of age.

Other than that - the most obvious reason for weightloss and excessive begging that I know - is intestinal worms! You could try out giving him a treatment for worms before you try anything else!

Love,
Sus and the Bellacat